PHA, also known as primary aldosteronism, is characterized by the overproduction of the mineralocorticoid hormone aldosterone being not a result of excessive renin secretion. Aldosterone causes increase in sodium and water retention and potassium excretion in the kidneys, leading to arterial hypertension. The diagnosis of PHA in patients with arterial hypertension is a significant analytical challenge due to the interference of currently available tests with anti-hypertensive treatments and the insufficient diagnostic power of the employed assays. PHA has many causes, including adrenal hyperplasia and adrenal carcinoma. When it occurs due to a solitary aldosterone-secreting adrenal adenoma, which is a type of benign tumor and is the most frequent cause of PHA (66% of cases), it is known as Conn's syndrome. Other causes of PHA include bilateral idiopathic adrenal hyperplasia (30% of cases), primary (unilateral) adrenal hyperplasia (2% of cases), aldosterone-producing adrenocortical carcinoma (<1% of cases), familial hyperaldosteronism (FH), glucocorticoid-remediable aldosteronism (FH type I, <1% of cases), FH type II (<2% of cases) and ectopic aldosterone-producing adenoma or carcinoma (<0.1% of cases) (Williams textbook of endocrinology. (11th ed.). Philadelphia: Saunders/Elsevier. 2008. ISBN 978-1-4160-2911-3.). However, due to the limited diagnostic capabilities, data about the prevalence of subforms of PHA are divergent. Recent studies indicate that the prevalence of aldosteronism due to bilateral idiopathic adrenal hyperplasia (IAH) is higher than had previously been believed, for as many as 75% of PHA cases. Once diagnosed, PHA can be usually cured by a surgical intervention.
Measuring aldosterone alone is not considered adequate to diagnose primary hyperaldosteronism. It is known that in contrast to measuring the aldosterone levels alone, the diagnostic specificity and sensitivity for detecting PHA can be improved by measuring renin activity or concentration and aldosterone and combining the two parameters to a arithmetic ratio, the aldosterone-to-renin ratio (ARR), which is currently used for diagnosis of PHA (Tiu S, Choi C, Shek C, Ng Y, Chan F, Ng C, Kong A (2005). “The use of aldosterone-renin ratio as a diagnostic test for primary hyperaldosteronism and its test characteristics under different conditions of blood sampling”. J Clin Endocrinol Metab 90 (1): 8. doi:10.1210/jc.2004-1149. PMID 15483077). The Aldosterone-to-renin ratio (ARR) is the mass concentration of aldosterone divided by the renin activity and/or renin concentration in blood plasma. The Aldosterone-to-renin ratio can be given in ng/dL per ng/(mL·h), that is, nanogram per deciliter of aldosterone per nanogram per (milliliter×hour) of renin. Also, it can be given in pmol/liter per μg/(liter·h), where aldosterone is given in molar concentration. The former can be converted to the latter by multiplying with 27.6. Also, the inverse value is occasionally given, that is, the renin-to-aldosterone ratio, the value of which is the multiplicative inverse of the aldosterone-to-renin ratio. Ratios between aldosterone and renin might also be calculated using other concentration units (mass unit per ml and/or amount unit per ml) for any of the two parameters resulting in different absolute values for the ratio while containing the similar information. The concentration of renin used for calculation of the ARR might also be given in μg UIE/ml, which is a unit frequently used in clinical diagnostics that also reflects the renin concentration.
The cutoff (or threshold) of normal individuals from those with primary hyperaldosteronism based on the ARR is significantly affected by the conditions of testing, such as body position and time of day. On average, an ARR cutoff of 23.6 ng/dL per ng/(mL·h), expressed in alternative units as 650 pmol/liter per μg/(liter·h), has been estimated to have a sensitivity of 97% and specificity of 94% (Tiu et al, cited above). An ARR value in an individual that is higher than the cutoff is used in the prior art to indicate primary hyperaldosteronism.
If the inverse ratio (i.e. renin-to-aldosterone) ratio is used, a value lower than the cutoff is considered to indicate primary hyperaldosteronism.
However, the broad range of ARR displayed by patients suffering from PHA allows no clear-cut and reliable discrimination between essential hypertension and PHA, thus leading to false-positive and/or false-negative diagnostic results and treatment decisions. Special medication and dietary requirements together with a sophisticated testing protocol involving saline infusion are required to improve the diagnostic power of the ARR in a confirmatory testing procedure subsequent to ARR screening.
It is suggested by endocrine societies to screen for PHA in patient groups at risk including patients with Joint National Commission stage 2 (>160-179/100-109 mm Hg), stage 3 (>180/110 mm Hg), or drug-resistant hypertension; hypertension and spontaneous or diuretic-induced hypokalemia; hypertension with adrenal incidentaloma; or hypertension and a family history of early-onset hypertension or cerebrovascular accident at a young age (<40 yr). Hypertensive first-degree relatives of patients with PHA show also an increased risk for PHA. According to clinical guidelines, the standard way to the diagnosis of PHA till the decision of the curing surgery is considered to be laborious and represents several risks for the patients.
The rationale behind the measurement of the ARR to diagnose PHA lies behind the physiological pathways responsible for aldosterone secretion in the adrenal cortex. Renin is a key enzyme of the renin-angiotensin-system (RAS) producing angiotensin I from angiotensinogen, which is converted to Ang II via other peptidases. Ang II is known to bind to AT1-Receptors (AT1R) leading to the secretion of aldosterone, which results in it's physiologic effects in the kidney and other tissues. Under healthy conditions, the RAS regulates plasma aldosterone levels. Under the condition of PHA, aldosterone production becomes partially independent of the RAS, meaning that renin is not further necessary to maintain aldosterone production. The measurement of the ARR tries to make use of this deregulation of renin and aldosterone. PHA patients usually have increased plasma aldosterone levels. Therefore, some investigators require elevated aldosterone levels in addition to elevated ARR for a positive screening test for PHA (usually aldosterone >15 ng/dl).
The diagnostic process for PHA is started with an ARR case detection test in patient groups specified above (John W. Funder et al.; J Clin Endocrinol Metab. September 2008, 93(9):3266-3281). In case this first measured ARR value exceeds a certain threshold, the patient is subjected to further testing in order to assure the validity of the obtained results. Of note, the exact value for the ARR threshold is still discussed in the literature, due to the frequent occurrence of false negatives and positives.
While there are few anti-hypertensive drugs that are thought to have only limited effects on the measured ARR value, many anti-hypertensive drugs are known to interfere strongly with ARR testing. The main cause of interference is represented by strong impact of these drugs on renin concentration and renin activity, leading to altered ARR results. As a consequence, a wash out phase of anti-hypertensive drugs is usually necessary before confirmation testing, which is a considerable risk for the hypertensive patients. Confirmation testing itself consists of a time consuming and cost intensive clinical procedure that is intended to reduce the renin levels of patients in response to osmotic or drug challenges in combination with ARR testing before and after the procedure.
A very common PHA confirmation test is a saline infusion test, where two liters of 0.9% saline is administered to the patient in the course of 4 hours. The volume increase should result in a decrease in renin activity and concentration. Post-test aldosterone levels below 50 pg/ml are thought to indicate the absence of PHA, while post-test aldosterone levels above 100 pg/ml are interpreted as a probable sign of PHA. Values between 50 pg/ml and 100 pg/ml are regarded to be indeterminate (John W. Funder et al.; J Clin Endocrinol Metab. September 2008, 93(9):3266-3281).
Positive confirmation testing triggers further clinical tests including adrenal imaging techniques, such as e.g. computed tomography (CT) and adrenal vein sampling (AVS) to determine the source of excessive and renin independent aldosterone production. Once the subtype is classified, unilateral adrenalectomy or treatment with mineralocorticoid receptor antagonists can be performed.
The key step in the diagnostic process is case detection in high-risk hypertensive patients. The ARR as a case detection test could easily show false positive and negative outcomes as among hypertensive patients the ARR value distribution in PHA patients was shown to overlap with the ARR value distribution of in non-PHA patients (Gary L. Schwartz and Stephen T. Turner; Clinical Chemistry 51, No. 2, 2005), which could easily lead to unnecessary mistreatment of patients. In case of false positives, these mistreatments can result in severe complications as a drug wash out phase of several weeks in a patient being hypertensive despite taking at least three anti-hypertensive drugs pose a significant risk of fatal cardiovascular events during this period of uncontrolled blood pressure. In addition to that, confirmation testing usually requires hospitalization and constant monitoring by physicians being time and money consuming for the patient and the healthcare system. In case of a false negative result, the PHA patient will continue to live with resistant hypertension, which has a fatal prognosis due to a strongly increased risk for life threatening cardiovascular events like strokes or heart attacks.
The present invention provides a method for the diagnosis of primary hyperaldosteronism in a subject, comprising obtaining a biological sample from the subject, measuring the aldosterone level and the Ang II level, and calculating the ratio thereof (aldosterone-to-angiotensin II ratio, AA2R). Said method has substantial advantages over the above described currently used ARR-based diagnostic methods.